Process for precise multichip integration and product thereof

ABSTRACT

Process for making an integrated circuit module and product thereof including a carrier supporting a plurality of precisely aligned semiconductor circuit chips having uniform thicknesses.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to manufacture of multichip integrated circuitdevice configurations.

2. Description of the Related Art

In the manufacture of integrated circuit devices, system levelintegration on a single highly wirable substrate is desirable forachieving higher system functionality and performance. Namely, it isdesirable to integrate high performance logic, dense memory, RFcircuitry, other analog functions, microelectro-mechanical function, andso forth, on a single substrate. However, the technologies used in theseindividual functions are diverse, and they employ highly varied basetechnologies, different costs and yields. While it is possible tointegrate a few of these components in a given semiconductor technology,such an approach requires compromises in technology with a concomitantdegrading of some of the technology attributes, e.g., a reduction ofDRAM density in a logic based merged logic and technology, or areduction of logic performance in a DRAM based merged logic and DRAMtechnology.

Advances occurring in semiconductor processing have permitted the scaleof individual integrated circuit devices to be shrunk to make itpossible to incorporate increasing amounts of functionality in a singleintegrated circuit chip. For example, sixteen 1 MBIT Dynamic RandomAccess Memory (DRAM) chips of equal size in 1984 required a total chiparea of 800 mm², while a single 16 MBIT design containing the samefunctionality in 1990 required only 110 mm². Thus, although theindividual chip size has increased by approximately 50%, the net chiparea has been reduced by a factor of 8. Accordingly, as integratedcircuit chips are required to incorporate more and more functionality,the size of the chips has been steadily increasing.

However, there are practical problems associated with continuallyincreasing the maximum chip size. A first set of problems relates to thephysical limits of present day fabrication equipment. For example,state-of-the-art manufacturing lithography equipment commonly used toexpose high resolution patterns through masks onto semiconductorsubstrates effectively limits chip size to the size of the lithographyexposure field of the equipment. The size of the direct exposure fieldof state-of-the-art manufacturing lithography equipment in the mid 1990sis generally on the order of 25 mm in diameter, allowing square chipdesign exposure of about 324 mm² (18 mm×18 mm). Most DRAM chip designsin development are rectangular and tend to be 20 mm×10 mm, or larger. Asolution to the exposure field/stitching problem is to developfabrication equipment having a larger exposure field and, therefore, thecapability to manufacture larger chips without stitching masks together.However, such a solution would require massive financial investment inresearch and development.

Another problem relates to the general trend of wafer yields decreasingwith increasing chip size. That is, it has been observed that as chiparea increases, the effective chip yields reduce nearly linearly. Thedecrease in yield with increasing chip size can be attributed to thefact that for the same quality of semiconductor, any defect existing ina larger chip results in an overall greater area waste than is the casewith smaller chip wafers. As chip size increases, the cost ofmanufacturing due to yield degradation becomes prohibitive.

Conventional multi-chip modules (MCM's) have avoided the problemsassociated with producing large chips by combining a plurality ofsmall-sized chips in a larger package. For example, U.S. Pat. No.4,489,364, assigned to IBM, discloses a ceramic chip carrier forsupporting an array of chips by means of solder balls, such ascontrolled collapsed chip connections (i.e., C4). However, such MCMstend to be extremely expensive due to their multilayered ceramicfeatures and they require significantly more area than the net overallarea of the combined set of chips. As a consequence, the wiring densityis not sufficiently high for current requirements in many cases.

Prior to the present invention, there was an unsatisfied need for amultichip integration scheme for combining, in close proximity, aplurality of semiconductor integrated circuit chips.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a manufacturingprocess for forming a very dense integrated circuit module withintegrated circuit chips and a substrate carrier assembled in aself-aligned manner.

It is another object of the invention to provide an ability to planarizea plurality of integrated circuit chips having non-uniform thicknessesfor attachment to a common substrate carrier.

It is yet another object of the present invention to provide a compositeintegrated circuit module with more precise chip positioning and tighterchip-to-chip placement on a permanent substrate carrier.

These and other objectives are achieved in the present invention byproviding a high level, low profile wafer level integrated system on asingle substrate carrier using silicon based technology.

The present invention uses a scheme for front reference placement of adiced component chip (DCC) that permits the retention of a planar wiringsurface. A mechanical interlock is provided between a three-dimensionaltopography formed in the surface of a temporary mandrel and athree-dimensional topography formed in the front surface of one or morecircuit chips which ensures maintenance of precise alignment of the chiparray during a backside planarization operation preformed on the chipsthat renders their thicknesses uniform before transferring the preciselyaligned chip array to a planar surface of a permanent carrier substrate.

In one embodiment, this mechanical interlock of the temporary mandreland circuit chips is accomplished by crystallographically anisotropicorientational etching of projection and recess surface profiles,respectively, in similar semiconductor bulk materials used in themandrel and chip integrated circuit materials. The juxtaposed slopingangled faces of the chip recess and the mandrel projection permitinsertion of mandrel projection into the chip recess until the opposingmajor flats of the chip and mandrel come into intimate contact with eachother.

In one embodiment, alignment of each chip on the mandrel preferablyinvolves a match-up between two or four pairs of juxtaposed slopingfaces, i.e., when the mandrel projection is inserted within a trench ora four-sided well, respectively, provided in the front face of thecircuit chip, preferably at its kerf area. This invention permits verydense packing of a plurality of variegated planarized circuit chips on acommon carrier substrate.

High performance DRAM and logic components are tested and integrated bythe inventive method. The present invention offers the advantage that itcan make use of commercially available substrates and wafer bondingtechnology and silicon back end (BEOL) technology.

In the chip module configurations made by the present invention, thewiring loads are significantly lower than packaging loads even thoughthe wiring loads are greater than conventional silicon wiring loads.

The inventive process permits erector-set type chip buildingopportunities. For instance, the invention provides for higher yieldingsmaller chips to be placed on a carrier substrate to permit generationof effectively larger composite chips with higher yields. Also,different chips can be optimized in standard (e.g., DRAM, SRAM, Logic,and so forth) process lines before their marriage with other chips fromother process lines, on a common carrier substrate, as opposed toattempting to generate one large chip containing all elements of thefinal chip. Using such smaller chips as "chip sectors" in this way, thechip sectors can be separately pre-tested for quality and functionalitybefore final placement on a common substrate, thereby optimizing finalchip yields. The present invention also allows for a mix and matchapproach to chip placement with each other on a carrier substrate torealize large chip hybrids (e.g., GaAs with SOI, DRAM with Logic, and soforth).

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description with references tothe drawings, in which:

FIG. 1A is an elevational view depicting locations of masked areas to beused in the formation of positioning guidance keys to permit andmaintain aligned association of integrated circuit chips with atemporary mandrel during processing according to the process of theinvention.

FIG. 1B is an elevational view depicting the manner of alignedassociation of integrated circuit chips with the temporary mandrel usingtheir respective mesa guide keys and cavities made according to theprocess of the invention.

FIGS. 2A-2D shows a process scheme for forming a recess in the chipsubstrate at the kerf areas where dicing can be performed to provideindividual chips with alignment cavities.

FIG. 3A is an elevational view showing an array of positioning guidancecavities formed in the front side kerf area of an integrated circuitchip.

FIG. 3B is an elevational view of a diced chip positioned on the mandrelby mechanical interlock of a cavity in the chip with a mesa on themandrel.

FIG. 3C is an enlarged view taken perpendicular to direction A--A ofarea Q indicated in FIG. 3B.

FIG. 3D is an enlarged cross-sectional view of a mesa-to-cavityinterlock according to one embodiment of this invention taken along thecenterline CL shown in FIG. 3B.

FIG. 3E is an enlarged view of a mesa-to-cavity interlock according toanother embodiment of this invention taken along the centerline CL shownin FIG. 3B.

FIGS. 4A-4E depict the overall scheme of process steps according to theinvention for making an assembly of planarized circuit chips processedto have uniform thickness.

FIG. 5 depicts a top view of hybrid chip module assembled according tothe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

With reference to FIG. 1A, the process of the invention begins byproviding a removable mandrel 1. The removable mandrel 1 is used as atemporary chip support in the inventive process scheme. The mandrel 1 isa semiconductor substrate, e.g., a silicon wafer, with at least themajor face thereof to be processed according to this invention polishedand thermally oxidized according to conventional practices. Thethickness tolerance is not critical, and the overall thickness may be asthin as may be reliably processed in the manner described herein. Thecrystallographic orientation is preferably (100). The mandrel 1 islightly doped (e.g., 1-10 ohm-cm) with an n-type or p-type dopant. Themandrel is lightly doped with such impurities so that it can beselectively etched away when it is time in the process to selectivelyetch-remove the mandrel from integrated circuit chips after they havebeen positioned and appropriately processed thereon in a mannerdescribed herein.

Alignment marks (not shown) for aligning a subsequent etch mask to beformed on the mandrel surface are provided by printing pattern marksonto a major face 10 of the mandrel 1 at otherwise non-used regions. Thealignment marks are used to align the etch mask next formed on themandrel surface 10, and the alignment marks are used to ensure that themask is aligned with the [110] direction. The alignment pattern markscan be made by conventional methods, such as by forming alignment forksaccording to the techniques described by M. Vangho and Y. Baecklund, J.Micromech. Microeng., vol. 6, pp. 279-284 (1996), which permit a rapidand precise determination of the crystallographic orientation in siliconwafers and which can be used to align subsequent masks. The alignmentforks permit alignment accuracies of ±0.5 degrees to be achieved.Alignment pattern marks are also printed onto an opposing major face ofeach chip wafer to be positioned upon the mandrel 1.

Then, the oxidized surface of the mandrel 1 is patterned to leavediscrete rectangular or elongated box structures 11 of silicon oxide atthe desired locations of the mandrel silicon surface 10. For example,photolithography is used to leave silicon dioxide masking areas 11 onthe silicon surface 10 corresponding to the locations desired for thegeneration of mesa structure 2 (see FIG. 1B).

Standard pattern etching is then performed which results in upraisedmesa structures 2, such as shown in FIG. 1B, being formed in the surface10 of the mandrel 1 after the surrounding exposed silicon isanisotropically etched away from the rest of the mandrel surface 10.That is, crystallographically anisotropic etching is performed on theexposed unmasked surface region 11' using masking pattern layer 11 as amask resulting in removal of surface matter from mandrel 1 at rateswhich differ depending on crystallographic direction. The preferentialetching is applied to the bulk portions of the mandrel 1. For thisetching step, the etchant preferably can be KOH, EPW(ethylenediamine/pyrocatechol/water), or a modified EPW such as(ethylenediamine/pyrocatechol/quinoxaline/water). Also, other usefuletchants include hydrazine, quaternary ammonium hydroxides, andethylenediamine solutions, such as disclosed in the J. Electrochem. Soc.(JECS), Vol. 139, No. Apr. 4, 1992, pages 1170-1174, which teachings areincorporated herein by reference.

As indicated in FIG. 1B, the use of the anisotropic etchant results in(111) oriented sidewalls 3 on the mesas structures 2 formed in thesurface 10 of the mandrel 1, as shown in FIG. 1B. The mesa structures 2are located as an array 7 in such a way on the mandrel 1 so thatprefabricated integrated circuit chips 4, 5 can be placed closelyadjacent to each other on the mandrel 1. Preferably four mesas 2 will bepositioned on the mandrel 1 to interlock with four cavities formed inkerf areas at four sides of each chip (4 or 5). For example, array 7includes a group of four separate, yet adjacent mesa separate structures2. It will be appreciated that the array of mesa structures 2 shown inFIG. 1B is representative and not meant to be limiting. Other arrayconfigurations of the mesa structures 2 also could be used as long aseach array is sufficient to prevent lateral displacements of thecorresponding integrated circuit chips 4,5 when positioned on themandrel 1 during processing. Also, the number of IC chips 4,5 shown inFIG. 1B is also chosen arbitrarily merely to facilitate theillustration.

To form cavities 6 in chips 4, 5 to receive the mandrel alignment mesas2, and all other IC chips desired to be integrated together andplanarized while positioned on the alignment mesas 2 of the mandrel 1,separate processing takes place on the semiconductor wafer or wafersbearing prefabricated IC chips 4, 5, i.e., active areas have beenfabricated in the surface of a semiconductor substrate die, but thechips 4, 5 have not yet been diced (separated) out.

Although the following description illustrates the sourcing of the ICchips 4 and 5 from the same original substrate, it will be understoodthat the chips 4 and 5 in FIG. 1B. could be derived from differentsubstrates.

The cavity formation in the IC chips is now discussed in more detailwith reference to the process scheme shown in FIGS. 2A-2D. As shown inFIG. 2A, the IC chips 4 and 5 initially form part of a larger commonsubstrate wafer 21 constituting chip workpiece 206. Wafer 21 is asemiconductor wafer, such as silicon. Kerf area 8, e.g., a silicondioxide or other insulator material, forms a non-active area betweenchips 4 and 5. The back-end-of-line (BEOL) processing metallization 41,51 has already been performed on the active areas 401, 501 formed inwafer substrate 21 of the chips 4, 5, respectively. The active areas401, 501 provided will depend on the type of integrated circuit devicesdesired, for example, bipolar, CMOS, biCMOS, optoelectronics, and soforth. After such devices are formed in the active areas, metallizationis provided by conventional metallization techniques to form connectionsbetween devices within the same active area. Preferably, additionalmetal layers (not shown), sufficient for enabling the testing of thefinished integrated circuit chip, are also deposited at this step. Suchtesting preferably provides the ability to screen out defectiveintegrated circuit chips after testing and prior to incorporating theminto a larger package, thereby enhancing overall yield of the ultimateassembly. After local metallization and testing, the BEOL metallization41, 51 for the active areas 401, 501 of the chips 4, 5, respectively,can optionally be coated with a temporary protective layer (not shown)to shield the active areas 401, 501 during further processing. Theprotective layer could comprise one of many polymers (for example,polyimide or other insulators), which are readily applied, are surfaceconformal, have adequate thickness (approximately 1-5 μm), and canwithstand subsequent planarization processing. After any such optionalprotective layer is placed on the metallization, the surfaces of thechip substrate material 21 is ready for processing according to thisinvention as described below.

Continuing in the description of the inventive process, and as shown inFIG. 2B, the kerf area 8 common between chips 4,5 of the front surface203 of the silicon substrate 21, is appropriately patterned to form maskpattern 202, e.g., a photolithographically patterned Si₃ N₄ film. Asshown in FIG. 2C, then the exposed area of the substrate 21 is wetetched to create the recess 20. The nature of this etch operation is setforth in greater detail hereinafter.

The chip 5 is formed in a single crystal semiconductor wafer substrate21 in which the mask 202 is aligned with the [110] direction of theupper surface 203 of the silicon substrate 21. A crystallographicanisotropic wet etch of the substrate 21, where exposed through mask 202is used to form recesses 20 comprised of beveled side edges 9terminating on a flat bottom 9' in the bulk of substrate wafer 21 in thekerf area 8 dividing the active areas 401 and 501 of neighboring chips4, 5. The side edges 9 of recess 20 form an angle α with substratesurface 203. While silicon nitride is preferred for mask layer 202, thepresent invention is not limited thereto, as other conventional maskmaterials used for anisotropic wet etching of silicon wafers also couldbe used.

The wet anisotropic etchants that can be used for etching cavities 20 inthe silicon substrate 21 located below the kerf area 8 between adjacentchips 4 and 5 include the same etchants used for etching the mesastructures 2 on the mandrel 1. These etchants, due to their anisotropicnature, have differential etch rates for different crystallographicplanes, thereby creating a tapered or beveled side edge in the recess orcavity created. Therefore, anisotropic etchant results in (111) orientedsidewalls 9 on the cavities 20 formed in the kerf areas 8 between chips4 and 5. For example, using KOH etchant on a (100) silicon wafer, theetch angle α will be 54.7° relative to the (100) plane.

The geometric dimensions of the unexposed (masked) silicon surface-areas203 have the important function of controlling the lateral dimensions ofthe beveled recess edges 9 of the cavities 20. Since the silicon etch isanisotropic, (111) crystal surface forms the planar recess bottom 9'.

In FIG. 2D, the mask 202 and remaining kerf area 8 has been removed sothat the extension of the angled surface 9 offers clearance access, asindicated by hatched lines "C" extending beyond the thickness "t" ofeach of the BEOL regions 41 and 51, for a mesa 2 of a mandrel 1. Thethickness of the PECVD layer 204 is formed sufficiently thin relative tothe thickness of the BEOL areas 41, 51 such that its thickness can bepractically disregarded insofar as the clearance C.

At this stage of processing any optional protective layer that has beenpreviously applied to the BEOL 41, 51, as described above, is removed.

As also shown in FIG. 2D, an important operation performed at this stageof processing is that the outer surface of the workpiece 206 is coatedwith a thin layer 204 of PECVD (plasma-enhanced chemical vapordeposited) SiO₂.

At this point, the substrate 21 is diced through at the centerline CL ofrecess 20 to separate chip 5 from chip 4, as shown in FIG. 3A. The dicedchips 4, 5 retain a major portion of each half of the geometry oforiginal cavity 20. The dicing will remove some minor portion of thecavity 20 by virtue of the thickness of the dicing wheel blade (unlesscleaving is instead used). However, the (111) oriented sidewall 9adjacent to the device active area 22 remains with the chip 5 (andsimilarly to chip 4).

The sum of the cavities 6 left in each of chip 5 and chip 4 (separatedfrom chip 5) are slightly less than one half the original width oforiginal recess 20 indicated in prior FIG. 2D. However, if conventionalcleaving is instead used to separate chips 4 and 5, the cavities 6 shownin FIG. 3A on chip 5, which were concurrently formed on severaldifferent sides of chip 5 in the manner described above, are essentiallythe same size as the original width of original recess 20.

The positions and number of cavities 6 in the kerf areas 8 can vary, asdo the length of them. However, the ultimate geometry, size and numberof cavities 6 in each kerf area 8 formed on and as part of theintegrated circuit chip 5 must be related to the geometry, size andnumber of corresponding mesa structures 2 formed on the mandrel 1, asexplained in detail hereinafter.

The key consideration is the manner in which the alignment cavities 6 inFIG. 3A have been formed in the kerf area of each IC chip 4, 5 to beintegrated together. In this regard, the objective is to form an etchedrecess 20 (e.g., see FIG. 2D) in the kerf area of a substrate wafer,which upon dicing down its centerline CL, provides a chip 5 having afrontside 201 with cavities 6 in the kerf area 8 bounding the chip'sactive area 501 and where each cavity 6 fits over a portion of a mesaprojection 2 of the mandrel 1 (see FIG. 3B).

The geometries involved with mating integrated circuit chip 5, forexample, to a mesa 2 on the surface of a temporary mandrel 1 are shownwith greater detail in the enlarged views of FIGS. 3C-E. Although onlythe chip 5 is shown as positioned on the mandrel 1 in FIGS. 3C-E forillustrative purposes, it will be understood that the present inventionencompasses positioning a single chip or a plurality of chips presentedon mesas 2 formed on the mandrel 1.

For this illustration, the chip cavity 6 is a four-sided wellconfiguration with a flat bottom 9', illustrated in cross-section inFIGS. 3C-E, while the mandrel mesa projection 2 is a 4-sided structurewith a flat plateau 3' that is inserted into cavity 6 of chip 5.

In this manner, each of chips 4 and 5 will have appropriate cavities 6formed in the silicon substrate 21 thereof with beveled edges 9 and abottom depth of each cavity 6 permitting placement of the side edges 3and height dimension of the mesa structures 2 formed on the mandrel 1within cavity 6 at least until the outer major planar flat 201 of thechips meets the outer major planar flat 10 of mandrel as shown in FIG.3C. As also indicated in FIG. 3C, a gap 208 remains between the mesaplateau 3' and the bottom 9' of cavity 6 of chip 5 when the major flats(201, 10) are brought into physical coplanar contact.

Referring to FIGS. 3C-E, it is imperative that the cavity 6 and mesa 2be sized to permit the major flats, i.e., front surface 201 of the chip5, and the outer surface 10 of the mandrel 1, to directly contact eachother in co-planar fashion before, or at the latest, simultaneous towhen, the mesa plateau 3' contacts the cavity flat 9' at its bottom. Thecavity sidewalls 9 and the opposing side edges 3 of the mesa 2 aresloped at the same angle to permit conformal contact sliding between thetwo.

In the embodiment shown in greater detail in FIG. 3D where themetallization 51 is stepped back from the mesa 2 (such as in theintermediate assembly formed as shown in FIG. 2D), the height d of themesa 2 from flat 210 is a smaller value than the combined depth d₁comprised of the cavity depth d₂ in the substrate 21 plus the thicknessd₃ of the overlying metallization 51. The hatched lines in FIG. 3Dindicate the ability for inserting the mesa 2 into cavity 6 in which thecavity sidewalls 9 conformally confront the side edges 3 of the mesa 2.

In an alternate embodiment of the invention shown in FIG. 3E, the edges"e" of metallization 51 and active area 501 of chip 5, where borderingcavity 6, is arranged to be flush with the side edges 3 of mesa 2 whenfully positioned within cavity 6. To accomplish this, the width x of themesa 2 is sized to be larger than the width x' of the opening "o" formedin the active area 501 and metallization 51 to define the entrance tocavity 6, i.e., the lateral distance between active areas 501 andoverlying metallization 51. This sizing arrangement locks the mesa 2inside cavity 6 with the sloped cavity edge 9 being in contact with thesloped side 3 of the mesa 2. Thus, by providing mesa side edges 3 andcavity sidewalls 9 at the same angle, and by providing a mesa height ythat is smaller than the combined depth y' of the cavity 6 plus thethickness of the metallization 51, the mesa 2 can also interlock withthe outer profile presented by chip 5 at the edges "e" of the BEOLmetallization 51 and active area 501 bordering cavity 6.

A combination of different sized chips and/or chips having differentshapes defined by their peripheral edges (e.g., mixtures of squares andrectangles, and so forth)-can be positioned on the common mandrel 1 inthe above manners, as long as the chip depressions and mandrel mesaprojections are dimensioned in compliance with the constrains describedherein.

Where chips 4 and 5 are intended to ultimately remain side-by-side inclose proximity to each other in a finished electronic package, thespacing of gap 32 can be reduced to a gap dimension convenient for gapfilling and dicing. If the gap 32 is made too small, it can be difficultto wick filler into the gap. This gap 32 can be filled by a materialhaving a flowable state to facilitate complete filling of the gap 32.The gap filling material should be selected as a material able totolerate the heat associated with additional subsequent metallizationoperations, e.g., polyimide, epoxy, or SOG.

As indicated by the hatched lines in FIG. 1B, and with the resultingassembly shown in FIG. 4A, the diced integrated circuit chips 4, 5 areeach placed face down, i.e., with active areas 401,501 facing mandrel 1,onto and against the corresponding located mesa structure 2 on themandrel 1 to which it is mated with a standard pick and place tool.Contact is made between the major surface 10 of the mandrel 1 and theexposed oxidized layer 204 previously formed on BEOL area 51 andcavities 6 of the chip 5. Van der Waals forces cause the two surfaces toattractively associate. A brief anneal can be applied for severalminutes at a low temperature (350-450°) by either a hot stage orlampheating to cause the diced component chips 4, 5 to enhance theadherence between the mandrel 1 and diced chips 4,5. The elevatedtemperature bonding is done after the mandrel wafer 1 is fully populatedwith the desired collection of integrated circuit chips 4,5.Alternatively, the integrated circuit chips 4,5 can be adhesively bondedto the mandrel 1 with the use of an appropriate adhesive, such as atemperature stable epoxy that can tolerate temperatures greater than400° C. Critical to the success of the bonding is surface cleanliness.Therefore, just prior to the association step, the mandrel 1 and theintegrated circuit chips 4, 5 preferably are subjected to a conventionalcleaning procedure.

Upon mating the chip cavities 6 with the mesa structures 2 on themandrel 1 in this manner, the position of the chip 5 is fixed forpurposes of further processing involving planarizing the backside of theintegrated circuit chips, as described hereinafter.

In any event, referring now to the process scheme shown in FIGS. 4A-D,after mating chip 5 and chip 4 to mandrel 1 and providing a gap filler32 (e.g., polyimide), the next step of the process is performingbackside planarization of the chips 4,5. Chip 4 has been processed in asimilar manner as chips 4 and 5 as described above insofar as thepositioning cavity formation, except that chip 4' is derived from a chipworkpiece other than workpiece 206 (e.g., see FIG. 2A). Separate chipsderived from the same or different chip workpieces generally will nothave identical thicknesses. For example, the thickness of thesemiconductor substrate wafer may vary between chips derived fromseparate workpieces and/or different active area and metallizationprocessing will cause variations in thickness between separate chips.Therefore, the back surface profile 31 is not co-planar with -all thechip backsides 31' and 31". Accordingly, at this stage of processing,the back surfaces 31' and 31" are simultaneously planarized using highspeed silicon polishing, e.g., conventional CMP, used in waferpolishing, resulting in the planarized backside 33 extending co-planarwith both chip backsides 33' and 33", as shown in FIG. 4B. Aftercompleting planarization, the overall remaining thickness of all theplanarized chips 4', 5 of uniform thickness can be as little as 200 μm,or even smaller.

Then, a permanent substrate carrier 34 is provided for chip bonding, asshown in FIG. 4C. The permanent substrate carrier is pretreated prior tocontact with the chips and mandrel assembly whereby a thin film (notshown) of PECVD silicon dioxide is formed on both its backside 36 andwhat will be its chip receiving face 33'. The backside oxide will beused as an etch stop while the frontside oxide formed on carrier 34 inthis manner will facilitate joinder to the chips 4, 5. The polishedbackside 33 of the bonded chips 4,5 is then placed in contact with theplanar, oxidized surface 33" of carrier wafer 34. The bonding process isthen repeated so that the bonded chips 4,5 at this time are sandwichedin-between the original mandrel 1 with its mesas 2, and the planarcarrier wafer 34, with the resulting assembly shown in FIG. 4C.

The low doped mandrel 1 is then selectively removed by immersing themandrel 1 in KOH, EPGW, or another anisotropic etchant for silicon, toremove the temporary support 1 from the chips 4,5, to leave chips 4,5exposed on the front outermost planar surface 201 having themetallization 41, 51 and active device areas 401, 501, respectively,while retaining the attachment of the back sides 33', 33", respectively,of the chips 4,5 to carrier wafer 34, and resulting in the structureshown in FIG. 4D. To reduce the processing time needed to remove themandrel 1, it is also possible to perform a preliminary operation on themandrel 1 in which it is mechanically lapped to remove a significantthickness of the mandrel 1 from its exposed side, and then lapping isdiscontinued when only a relatively small thickness of the mandrel 1remains for etch removal. PECVD silicon oxide layer 204 that waspreviously formed on the surfaces of the chips 4 and 5 provides anatural etch stop to protect chips 4 and 5 from the etching operationused to remove the mandrel 1. Likewise, the PECVD silicon oxide layerformed on the back face 36 of the permanent substrate 34 protects itfrom the etchant used to remove mandrel 1. The mandrel 1 was low dopedto be selectively etched at a higher rate from a productivity standpointsince a highly doped mandrel, which could be used, will etch relativelymore slowly.

The removal of the mandrel 1 in this manner will leave behind, in thoseareas where the mesas 2 were positioned into the cavities 6, an open setof cavities. These must be filled and planarized with an appropriateinsulating material 35, such as polyimide, as shown in FIG. 4D.

To provide inter-DCC wiring, the exposed front surface 21 of thecollection of chips 4,5 is subjected to conventional BEOL interconnectmetallurgy processing to provide BEOL interconnect 205. This can beaccomplished due to the accuracy of the earlier step of chip positioningonto the temporary mandrel 1. Vias are cut through the outer oxide layeron the front surface of the chips 4,5, filled with metal and polisheddown as in standard via-stud processing. If the line width and pitchpermit, both lift off and sub-etch process may also be used. In order toaccommodate the tolerances of DCC placement, a set of buffer padsbetween the DCCs may be employed as intermediate landing sites for theinter DCC wiring. Multilevel metallization is formed by conventionalmethods to complete the wafer level wired system shown in FIG. 4E.

In this manner, several DCCs with different functionality can be wiredwith line widths, pitches and distances that are characteristic of abackend process rather than a MCM process.

FIG. 5 depicts a top view of an example of hybrid chip package that canbe assembled according to the invention. Using the chipdepression/mandrel projection system of this invention duringsimultaneous backside planarization of a plurality of differentthickness chips, a variety of different types of integrated circuitchips and sizes can be assembled together on a common substrate. Asillustrated in FIG. 5, the final integrated module 500 has variegatedchips 503a-e comprised of ROM, MCU, DSP, SRAM and FLASH chips positionedon a common carrier substrate 502. It should be noted that theintegrated circuit chips 503a-e, which are assembled together in themodule 500, are not limited to the examples provided here, and they maybe formed from substrates of similar or dissimilar materials, and theprocessing methods to form the active devices in the integrated circuitchips may have been radically different from each other as long as thechip positioning means of the present invention is employed.

The mandrel projections 2 are illustrated in FIG. 1B as four-sidedmesa-like structures having four sloping sidewalls. It can be understoodthat if an etch mask stripe is provided to extend partly from one edgeof the mandrel wafer 10 towards an opposite edge thereof that aridge-like projection would be created by the etching procedure insteadof a mesa. This embodiment would prevent movement of a chip positionedon the ridge in lateral direction to the ridge, although the chips couldslide along the ridge. Therefore, in this optional scenario, the ridgeshould be made discontinuous with a terminus provided at both endsbefore the ridge reaches either edge of the mandrel wafer to afford someultimate sliding movement control for chips positioned thereon. Such aridge-like projection on the mandrel could be used where a trench-likedepression (i.e., two spaced beveled edges extending along a common flattrench bottom) is formed on the bottom surface of a chip from one edgethereof to the other for chip positioning purposes.

Also, while the semiconductor wafer material used for chips 4 and 5, andthe mandrel 1, have been exemplified as silicon, these materials alsomay be other semiconductor materials such as germanium, galliumarsenide, a compound of a Group II element and a group VI element, acompound of a Group III element and a group V element, or any othersemiconductor material susceptible to crystallographic anisotropicetching. The wafer thicknesses used in practicing this invention can bea standard wafer thickness in the industry, such as about 700 to 800 μmfor a 200 mm diameter wafer, although it is not limited thereto, as longas the mandrel and chips must have sufficient thickness to allow thealignment mesas and cavities, respectively, to be formed in the waferthicknesses provided.

In the implementation of this invention, conventional chip handling andplacement tools, such as robotic placement chucks and vacuum probescapable of controlled x-y movement, may be used to position anintegrated circuit chip fabricated according to this invention such thatits surface depression is interfitted with a corresponding surfaceprojection provided on the mandrel surface. Once the circuit chip isproperly oriented, a robotically controlled vacuum probe will place thechip onto mandrel mesa projections, the orientation of which has beensimilarly aligned. Once in place, the chips are mechanically agitated toinsert fully onto the mandrel mesa projections.

Optionally, one or more pressure relief holes (not shown) can be drilledthrough the mandrel, preferably prior to assembly, in order to relievepressure created by trapped gases. Suitable drilling techniques includelaser ablation, ion beam milling or cryogenic plasma etching.

The present invention enjoys various advantages over prior art schemes.The present invention permits the accommodation of chip modules that areof different thicknesses by use of the front surface leveling scheme.The front reference placement scheme of this invention allows theretention of a planar wiring surface. The result is a high level, lowprofile wafer level integrated system on a single low cost substratethat can use silicon based technology.

Also, since the respective depressions and projections provided in boththe integrated circuit chips and the temporary mandrel are definedlithographically, and created with anisotropic etching methods, therespective dimensions are highly controllable. The method also reducesthe overall areal size of the assembly due to smaller spacerequirements.

Still another advantage is improved overall chip yields of such chipassemblies over single large area chips having similar functionality.The improved yield derives from the generally superior yields of smallerchips coupled with pretesting of such chips prior to final assembly.

Still another advantage is the ability to assemble dissimilar integratedcircuit chips (e.g. silicon, gallium arsenide, etc.), requiringdifferent processing conditions, into a final chip assembly.

While the invention has been described in terms of a few preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

What is claimed is:
 1. Process for making an integrated circuit modulein which a plurality of integrated circuit chips are assembled on acommon carrier substrate in precise alignment, comprising the stepsof:providing a mandrel having a planar mandrel surface and projectionsdistending from said planar mandrel surface, and said projections havingsloping sidewalls; providing a plurality of integrated circuit chipseach having a semiconductor substrate, a backside surface, and asubstantially planar frontside surface including active devices, whereinsurface depressions are formed through said frontside surface and intosaid semiconductor substrate, wherein each of said surface depressionshave beveled side edges with substantially the same angle of inclinationas that of said sloping sidewalls of one of said mandrel projections;placing one of said depressions of said chip onto said mesa of saidmandrel until said chip frontside surface contacts said planar mandrelsurface, whereby each chip is positioned on a projection; planarizingsaid backside surfaces of said chips; attaching said planarized backsidesurfaces of said chips to a permanent chip carrier; and removing saidmandrel from said frontside surfaces of said chips.
 2. The process ofclaim 1, wherein said mandrel projection has a height that is smaller indistance than the depth of said cavity.
 3. The process of claim 1,wherein said placing step comprises positioning said beveled edges ofsaid chip depressions and said sloping sidewalls of said mandrelprojections in direct conformal, juxtaposed contact.
 4. The process ofclaim 1, wherein said placing step comprises positioning said bevelededges of said chip depressions and said sloping sidewalls of saidmandrel projections in a spaced orientation relative to each other, andwherein a mechanical interlock occurs where said mesa physically abutsthe active area of said chip where bordering said depression.
 5. Theprocess of claim 1, wherein said mandrel and said chips are siliconwafers, and said mandrel projections and said chip depressions areformed by crystallographic anisotropic etching.
 6. The process of claim5, wherein said crystallographic anisotropic etching is performed usinga wet etchant.
 7. The process of claim 6, wherein said wet etchant isselected from KOH, ethylenediamine/pyrocatechol/water orethylenediamine/pyrocatechol/quinoxaline/water.
 8. The process of claim1, wherein said mandrel is lightly doped with impurities, and saidmandrel is selectively removed by etching.
 9. The process of claim 1,wherein said mandrel further includes:an upper face comprised of aplanar base surface portion bordering said projection wherein saidprojection has a flat top surface, wherein said sloping sidewall of saidprojection extends from said planar base surface portion to said flattop surface of said projection, and wherein said circuit chips furtherinclude:a front side surface comprised a planar inner chip surfaceportion bounded by a kerf region including said surface depressions andsaid surface depressions each has a planar recess bottom surface,wherein said beveled edges of said surface depressions each extend fromsaid planar outer chip surface portion to said planar recess bottomsurface.
 10. The process of claim 9, wherein said planar chip surfaceportion rests in direct contact on said planar base surface portion, anda vacant space exists between said recess bottom surface and said flattop surface of said projection.
 11. The process of claim 1, wherein saidsurface depression in said chip is formed with at least two bevelededges and said mandrel projection is formed with at least two slopingsidewalls, wherein said at least two beveled edges of said surfacedepression in said chip are in juxtaposition with said at least twosloping sidewalls of said mandrel projection.
 12. The process of claim1, wherein said surface depression in said chip is formed having fourbeveled edges and said mandrel projection is formed having four slopingsidewalls, wherein said four beveled edges of said surface depression insaid chip being in juxtaposition with said four sloping sidewalls ofsaid mandrel projection.
 13. The process of claim 1, wherein thematerial of at least a bulk portion of said mandrel and the material ofat least a bulk portion of said chips are selected as having the samecrystallographic structure.
 14. The process of claim 13, wherein saidmaterial is a semiconductor material.
 15. The process of claim 14,wherein said material is silicon.
 16. The process of claim 15, whereinsaid silicon is a (100) wafer, and said planar outer chip surfaceportion is aligned with the [110] direction and said beveled edge is a(111) oriented crystal face, and said flat top surface of saidprojection is aligned with the [110] direction and said sloping sidewallis a (111) oriented crystal face.
 17. The process of claim 16, whereinsaid sloping sidewall of said projection forms an angle of approximately125.3° with said flat top surface of said projection, and said bevelededge forms an angle of approximately 125.3° with said planar outer chipsurface portion.
 18. The integrated circuit module product as made bythe process of claim 1.